Color Centers in KI:T1 X-Rayed at Low Temperatures

Abstract

The optical properties of the phosphor KI:Tl have been studied after x-raying at 5 or 78°K. The interactions among the several simultaneously produced centers have been studied by following and correlating changes in luminescent intensity and optical absorption spectra as the crystal is warmed, and an explanation of these effects is offered in terms of retrapping and recombination. Perturbation bands due to the modification of the energy levels of the luminescent center as a result of the interaction with a color center are observed. The major centers produced in the more heavily doped crystals by brief irradiation at low temperatures do not involve vacancies and are identified as an electron center associated with Tl, I2—, and V centers near Tl+. The optical absorption spectrum of I2— consists of bands at 404 and 800 mμ. (The absorption of Br2— in KBr:Tl incidentally has also been identified.) On the basis of certain similarities in luminescence properties, essentially similar effects are proposed for ultraviolet as for x-ray excitation.